Why will my sending unit not shut off?

[Lonesome715]

I took my 87 Air Force deuce for a drive around the block today. It has been sitting for a couple of months at least. When I returned I noticed the sending unit was still running. I gave it a little while to see if it will shut off on its own and it did not. I had to end up unplugging it. What causes this problem and how can I repair it?

 

[mhassett]

I have an 88 Air Force duece. What sending unit are you talking about? What did you unplug? Need more info.
mhassett

 

[1qwkgoat]

I'm going to assume he means the fuel pump.

 

[cbvet]

 

[DieselBob]

That would tend to indicate a bad master switch. The fuel pump is driven directly from the switch unless there is a difference in the AF version of the electrical system. Do the oil pressure and water temp go to zero when the master switch is off.

 

[Lonesome715]

Sorry about that. I was not thinking. I am referring to the fuel sending unit. O pulled the three prong lug on top of the fuel tank. I did not noice if my guages were off or not. Which master switch are you referring to?

 

[Capt.Marion]

The main power switch on the dash with the long, funky lever.

 

[DieselBob]

Number 2 in the figure. Some also refer to it as the ignition switch

 

[Lonesome715]

That is what I thought you meant. It is turned off and the sending unit still runs.

 

[DieselBob]

According to the wiring diagram I have it would indicate that for some reason your master switch is not breaking the connection between terminal "A" and "B" inside the switch. Terminal "A" is circuit #11 that is the hot feed into the master switch and terminal "B" is the feed out of the switch, circuit #27 to the instrument panel, starter switch, light switch, etc including circuit #654 to the fuel pump.

 

[Lonesome715]

I wonder why and how that would happen.

 

[DieselBob]

Will the starter button work with the master switch in the off position ?

 

[Lonesome715]

I will go find out.

 

[Lonesome715]

Well, all seems fine now. No the starter did not kick on with the master switch off. But I did go ahead and start the truck and let it idle for a few minutes. I then shut it down and the sending unit shut down with it.

 

[DieselBob]

You got me on that one. It is a very simple circuit and I don't see how it could back feed from another circuit. If you want to send me a PM with your email address I can send you the wiring diagram if you like.

 

[Lonesome715]

I have no idea either. I know strange things happen, but ****. I wonder if it will do it again. I do not start or drive this truck very often because I cannot afford to get it legal.

 

[frodobaggins]

It may be bypassed for some reason. Time to start tracing wires with the multimeter.

 

[Lonesome715]

I wish I had one. Other than a visual check, is there another way I could check them?

 

[dabtl]

Multi-meters are about $10 at Lowe's

 

[Lonesome715]

I cannot spare it right now. Later on when I can though that sounds like a good deal. Are the good quality for that price?